This application relates to compensation for and control of optical dispersion, and more specifically, to techniques for reducing polarization-mode dispersion in optical media such as optical fibers used in various fiber systems including optical wavelength-division multiplexing (WDM) fiber systems.
Some optical transmission media, e.g., optical fibers, may be optically birefringent and hence exhibit different refractive indices for light with different polarizations along two orthogonal principal directions. Therefore, an optical signal, comprising of two components along the two orthogonal principal directions for each frequency, can be distorted after propagation through the transmission medium because the two components propagate at different group velocities. One effect of such dispersion-induced distortion is broadening of optical pulses. This polarization-dependent optical dispersion is generally referred to as the first-order polarization-mode dispersion (xe2x80x9cPMDxe2x80x9d). The degree of the first-order PMD may be approximately characterized by the average differential group delay (xe2x80x9cDGDxe2x80x9d) between two orthogonal principal states of polarization. PMD also exhibits higher order effects and further complicates the compensation and management of the PMD in fiber systems.
Typical causes for such birefringence in fibers include, among others, imperfect circular core and unbalanced stress in a fiber along different transverse directions. Notably, the direction of one axis of birefringence of the optical fiber may change randomly on a time scale that varies, e.g., from milliseconds to hours, depending on the external conditions. Thus, the DGD in an actual PMD fiber is not a fixed value but a random variable. In many fiber links in communication systems, the PMD-induced DGD has a Maxwellian probability density function.
Such polarization-mode dispersion is undesirable in part because the pulse broadening can limit the transmission bit rate, the transmission bandwidth, and other performance factors of an optical communication system. In fact, PMD is one of key limitations to the performance of some high-speed optical fiber communication systems at or above 10 Gbits/s per channel due to the fiber birefringence. It is desirable to control, manage, or compensate such optical dispersion in fiber systems.
This disclosure includes techniques for controlling, managing, or compensating both first-order and higher-order PMD in a two-stage PMD control scheme. The first PMD control stage includes a fixed PMD element designed to produce a fixed DGD for controlling second-order and higher-order PMD in the input optical signal. The second PMD control stage includes a variable PMD element designed to produce a variable DGD for controlling the residual first-order PMD in the input signal after being processed by the first stage. An optical element is interconnected between the fixed PMD element and the variable PMD element to rotate the polarization of the output light from the first stage by a desired angle.